LAN sharing internet connection?

Have succesfully (after a little help from here) connected my desk top to lap top. (Win XPpro on desktop, home on laptop)

Am using USB ADSL modem to connect with desktop. My laptop can make or break an internet connection through/on the desktop, and detects everytime it is connected, but is unable to display any webpages - comes up with "page cannot be displayed".

Whilst trying to display pages something flashes up on my desk top pc, but so quickly I can't identify it. Must be a box wrongly checked, but have not been able to find it.